1. Field of the Invention
The present invention relates to a connection structure and a connection member for electrical connection of power cables.
2. Description of Relevant Art
The power cable is constituted with: a cable conductor (hereafter sometimes simply called xe2x80x9cconductorxe2x80x9d) configured as a bundle of core wires for transmission of required electric power; a cable insulator (hereafter sometimes simply called xe2x80x9cinsulatorxe2x80x9d) configured for insulation about the conductor and made of, for example, a plastic material such as bridged polyethylene; a cable screen (hereafter sometimes simply called xe2x80x9cscreenxe2x80x9d) configured for electric screening about the insulator, and a cable outer cover (hereafter sometimes simply called xe2x80x9couter coverxe2x80x9d or xe2x80x9ccoverxe2x80x9d) configured for coverage and protection about the screen.
It therefore is necessary for electrical connection of power cables to employ a connection structure that has, as portions thereof, respective connection elements adaptive for power transmission, insulation, electric screening, and coverage and protection, and is provided with related considerations, in particular, for ensured prevention of breakdown due to insulation breakage at a connection portion for insulation.
In this respect, for power cables transmitting medium-voltage or high-voltage power, having great potential differences between a connection portion for power transmission and a connection portion for electric screening, there is needed a severe check for possible insulation breakage due to local concentration of electric field or stress.
This is specifically discussed below.
FIG. 9 illustrates a conventional connection structure CN100 employed for electrical connection between a pair of medium- or high-voltage power transmitting and plastic-insulated power cables PC1 and PC2, showing a longitudinal section at one side about an axis C.
The connection structure CN100 includes: a power transmitting connection portion (hereafter sometimes called xe2x80x9cpower transmitting portionxe2x80x9d or xe2x80x9ctransmission portionxe2x80x9d) 120 configured for electrical connection between stripped conductors 2 and 2 (more specifically, conductor ends, like intra) of power cables PC1 and PC2 at the left and right; a screening connection portion (hereafter sometimes called xe2x80x9cscreening portionxe2x80x9d) 130 configured for electrical connection between stripped screens 6 and 6 of power cables PC1 and PC2; an insulating connection portion (hereafter sometimes called xe2x80x9cinsulating portionxe2x80x9d) 140 filled between the transmission portion 120 and the screening portion 130 and configured to conformingly fit on stripped plastic insulators 5 and 5 of power cables PC1 and PC2; and a covering and protecting connection portion (hereafter sometimes called xe2x80x9ccovering portion) 150 configured, as a two-piece separable protection case in the figure, to water-lightly fit on outer covers 7 and 7 of power cables PC1 and PC2, covering outer periphery of the: screening portion 130.
Like later-described corresponding embodiments of the invention, the covering portion may be configured, not simply as a two-piece separable protection case, but also as a structure in which a thermally shrinkable tube made of polyethylene or equivalent is fit water-tight on the screening portion, or as a structure in which the screening portion is covered by a glass-fiber reinforced epoxy resin tube, with a compound filled water-tight in between.
The transmission portion 120 is configured with a tubular conductor 8 fit on ends of the cable conductors 2 and 2, a layer 9 of conductive rubber tape wound-fit on the tubular conductor 8 and remaining stripped parts of the cable conductors 2 and 2, and a buried relatively thick inner electrode 121 fit conforming on the wound layer 9 of rubber tape and ends of the cable insulators 5 and 5.
The internal electrode 121 is molded by filling a semi-conductive rubber material in a dedicated die therefor, as a substantially tubular member having at both ends thereof axially protruding outer peripheral parts 121a and 121b. 
The screening portion 130 is configured as a tubular outer electrode 131 with tapered end faces 131a and 131b at both ends, which is molded by filling a semi-conductive rubber material in a dedicated die therefor.
The insulating portion 140 is configured as a tubular insulating member 141 with tapered end faces 141a and 141b at both ends, which is made, by filling a polyethylene propylene base rubber material in a dedicated die with preset molds of inner and outer electrodes 121 and 131, as an integral pre-molded connection member PM100.
Like later-described corresponding embodiments of the invention, the insulating portion may be prefabricated, before filling a semi-conductive rubber material to form the outer electrode
As a desideratum, there has been needed a power cable connection structure of connection member allowing reduction of cost in such manufacture.
The connection structure CN100 of FIG. 9, employed for medium- or high-voltage power transmission, has great potential differences developed between the inner electrode 121 and the outer electrode 131, and needs severe prevention against breakdown of insulation member 141 due to electric stress concentration. It therefore has structures shown in FIG. 10A and FIG. 10B.
FIG. 10A and FIG. 10B are details of part-A and part-B of FIG. 9, respectively.
In the connection structure CN100, for prevention of insulation breakdown, each end of inner electrode 121 (FIG. 10A simply shows the right end 121b.) has an edge thereof rounded by a relatively large radius R as shown in FIG. 10A, to be 10 mm or more thick at the end, resulting in a radius difference exceeding 30 mm between the inside diameter of inner electrode 121 and the outside diameter of outer electrode 131 in FIG. 9, i.e., the thickness of pre-molded connection member PM100, of which reduction also has been a desideratum.
Further, in the connection structure CN100, as shown in FIG. 10B, the outer electrode 131 is formed as an electric field controlling stress cone large of radius to simply conform to the cable screen 6, resulting in a relatively large dimension in thickness as well as in length thereof, leading to a long size of connection.
The present inventors made eager investigations in view of the foregoing points, to find that a rubber electrode layer constituted as a sheet-shaped semi-conductive rubber layer covered (i.e. coated or laminated) at a prescribed region thereof with a rubber layer high of permittivity (i.e. dielectric constant) can be used as a screening electrode in a power-transmitting portion and/or an electric screening portion to control the concentration of electric stress in insulator within a vicinity of the covered region, to a significant degree.
In addition the high-permittivity rubber layer can achieve the control of electric stress concentration at a successfully effective level in practice, providing that it has a high specific permittivity (xcex5h) equal to or greater than a specific permittivity (xcex5c) of cable insulator times five. For example, for a typical xcex5c=2.3, the high-permittivity xcex5h may well be 15 to practically achieve a definite reduction in thickness of pre-molded connection member. It is noted that this condition (xcex5h=15) is met in any of later-described embodiments of the invention.
The content of find is illustrated for particular cases in FIG. 11A and FIG. 11B, where like elements of FIG. 10 are designated by like reference characters to omit redundancy.
FIG. 11A illustrates a connection structure in which a rubber electrode layer RE constituted as a sheet-shaped semi-conductive rubber layer SC covered at an outer periphery thereof with a high-permittivity rubber layer HP is interposed between a rubber tape wound layer 9 of a power transmitting portion and a rubber layer EPR of an insulating portion, and extended on an outer periphery of a cable insulator 5, whereby it is allowed to terminate an end SCa of the semi-conductive rubber layer SC by an extremely small radius r.
FIG. 11B illustrates a connection structure in which a rubber electrode layer RE constituted as a sheet-shaped semi-conductive rubber layer SC covered at an outer periphery thereof with a high-permittivity rubber layer HP is interposed between a cable screen 6 and a rubber layer EPR of an insulating portion, and extended on an outer periphery of a cable insulator 5, whereby, like the case of power transmitting portion described above, it is allowed to terminate an end SCa of the semi-conductive rubber layer SC by an extremely small radius r. Also the thickness of a semi-conductive rubber make outer electrode SCo can be extremely small.
For medium/high-voltage power oriented CV cables, the radius r of termination can be preferably set to r=0.5 mm or more.
The sheet-shaped semi-conductive rubber layer SC can be preferably configured at a side part thereof to lift off, i.e., to come up at a distance from the cable insulator 5, penetrating (see FIG. 6) or extending (see FIG. 7) to the interior of a corresponding part of the high-permittivity rubber layer HP. In this case, the length by which the side part of semi-conductive rubber layer SC penetrates or extends, or in other words, the length (L1 in FIG. 6 or FIG. 7) by which an inner part of high-permittivity rubber layer HP extends inside the semi-conductive rubber layer SC of a tubular form and contacts on the cable insulator 5 can be preferably set to 10 mm or less; and an axial extension or remaining width of the corresponding part of high-permittivity rubber layer HP, or in other words, the length (L2 in FIG. 6 or FIG. 7) or distance from a distal end of rounded part of semi-conductive rubber layer SC to an edge at end of high-permittivity rubber layer HP can be preferably set within a range over 5 mm, to an adequate dimension to be determined by an electric field analysis so that the electric stress concentration is kept under a critical value.
For a certain range of power to be transmitted, the semi-conductive rubber layer SC can be left without round termination.
The present invention is made with such points in view. It therefore is an object of the invention to provide on the basis of investigation results described, a connection structure and a connection member for electrical connection of power cables adapted for effective reduction in cost of manufacture and dimensional difference described.
To achieve the object, according to an aspect of the invention, a connection structure for electrical connection of power cables comprises a transmission portion having a first rubber electrode configured for electrical connection of a cable conductor, a screening portion having a second rubber electrode configured for electrical connection of a cable screen, and an insulating portion having a rubber insulator configured for insulation between the first and second rubber electrodes, and at least one of the first and second rubber electrodes comprises a rubber electrode layer configured with a sheet-shaped semi-conductive rubber layer, and a high-permittivity rubber layer covering at least one side of a peripheral part of the semi-conductive rubber layer.
According to another aspect of the invention, a connection member for electrical connection of power cables comprises a first rubber electrode configured for electrical connection of a cable conductor, a second rubber electrode configured for electrical connection of a cable screen, and a rubber insulator configured for insulation between the first and second rubber electrodes, and at least one of the first and second rubber electrodes comprises a rubber electrode layer configured with a sheet-shaped semi-conductive rubber layer, and a high-permittivity rubber layer covering at least one side of a peripheral part of the semi-conductive rubber layer.
According to those aspects, a rubber electrode layer having a sheet-shaped semi-conductive rubber layer covered with a high-permittivity rubber layer on at least one side of a peripheral part of the semi-conductive rubber layer is used as a rubber electrode of a power transmitting portion or of a screening portion, and controls the electric stress concentration in a rubber insulator at least within a vicinity of the peripheral part.
Accordingly, an end face of the rubber electrode is allowed to vary within a range of curvature under an increased upper limit, or within a range of radius of curvature over a decreased lower limit, with a commensurate contribution for the connection member to be thick, or for the condition of manufacture to be moderated, with decreased cost.